1. Field of the Invention
This invention generally relates to television receivers or other video signal reproducing devices, and more particularly is directed to an improvement in such devices for obtaining reproduced images with increased sharpness.
2. Description of the Prior Art
When an image is reproduced on the phosphor screen of a cathode ray tube in a television receiver or the like, the electron beam current in the cathode ray tube increases at a high luminance portion of the image so that the size of the landing spot of the electron beam on the phosphor screen becomes large and hence the sharpness of the reproduced image is inevitably lowered.
By way of example, when a video signal is changed between black and white levels, with relatively sharp rising-up and falling-down or transient changes of the level of the video signal, the size of an electron beam landing spot on the phosphor screen corresponding to a point where the video signal is risen up to the white level and a point where the signal is falling down from the white level becomes relatively large. Therefore, a corresponding portion of the image on the phosphor screen has a skirt like shape in the horizontal direction and hence the marginal portion of a bright area of the image is undefined or becomes vague to lower the sharpness thereof. For example, if a rectangular bright image portion of a predetermined size is desired to be reproduced on the phosphor screen of the cathode ray tube at its substantial center by a so-called window signal, as mentioned above, the size of the landing spot of the electron beam on the phosphor screen becomes large at the horizontal and vertical margins of the rectangular bright image portion. Thus, the bright image portion is expanded in both the horizontal and vertical directions with the result that its contour is made vague.